1. Field of the Invention
This invention relates to a vibration isolator provided between a vibration source and a base to control transmission of vibrations or from the vibration source to the base. The present invention is particularly suitable to an electronically controlled engine mount capable of changing damping characteristics according to conditions of vibrations input from an engine, vehicle driving conditions and other factors.
2. Description of the Related Art
As is well known, an engine is mounted on a body of a vehicle through an engine mount to prevent transmission of vibration from the engine to the vehicle body and to prevent transmission of engine shakes caused when the vehicle is abruptly accelerated or decelerated, or when the vehicle is started or braked. For example, U.S. Pat. No. 4,693,455 discloses an engine mount designed mainly to prevent transmission of vibration from an engine to a vehicle body.
FIG. 19 is a cross-sectional view of a conventional engine mount. As shown in FIG. 19, a housing 301 of the engine mount is generally cylindrical shaped and has an upper opening closed with a stay 303 and an elastic member 302 made of rubber and interposed between an opening portion and the stay 303. A partition member 304 in the form of a disk is positioned and fixed in the housing 301 to form a main fluid chamber 305 on the stay 303 side of partition member 304. A non-compressive fluid is enclosed in the main fluid chamber 305. A central portion of the partition member 304 is recessed to form an air chamber 306 in which a moving member 308 around which a moving coil 307 is wound is provided, and a permanent magnet (ferrite magnet) 309 in the form of a ring is provided around the moving member 308 so as to face the moving coil 307. The moving coil 307, the moving member 308 and the permanent magnet 309 constitute a voice coil 310. A sheet-like member 311 made of rubber is bonded to an upper surface of the partition member 304 so as to close the air chamber 306. The sheet-like member 311 prevents the non-compressive fluid in the main fluid chamber 305 from flowing into the air chamber 306. The moving member 308 is bonded to a lower surface of the sheet-like member 311. The moving member 308 is allowed to move slightly in a direction along an axis L of the engine mount, while its movement in the direction perpendicular to the axis L is limited.
A diaphragm 312 is positioned and fixed under the partition member 304. A sub fluid chamber 313 in which the non-compressive fluid is also enclosed is formed between the partition member 304 and the diaphragm 312. The sub fluid chamber 313 communicates with the main fluid chamber 305 through orifices formed in a peripheral portion of the partition member 304. As is well known, a damping effect is achieved when the non-compressive fluid passes through the orifices 314.
The thus-constructed engine mount is interposed between an engine and a vehicle body, with its housing 301 fixed to the vehicle body, and with a bolt 303a, which is welded to an upper surface of the stay 303, fixed to the engine. If the elastic member 302 is bent in a vertical direction by vibrations from the engine, periodic vibrations are input to the non-compressive fluid in the main fluid chamber 305. At this time, an alternating current having the same period as the vibrations is caused to flow through the moving coil 307 to forcibly vibrate the moving member 308 so that vibrations having a phase opposite that of the vibrations from the engine are input to the non-compressive fluid in the main fluid chamber 305. In this manner, the dynamic spring constant of the engine mount is reduced to shut off vibrations from the engine.
In the above-described engine mount, vibrations of the moving member 308 are transmitted to the non-compressive fluid in the main fluid chamber 305 through the sheet-like member 311, because a liquid seal is maintained by the sheet 311 between the main fluid chamber 305 and the air chamber 306. That is, the sheet-like member 311 made of rubber acts to dampen the vibration of the moving member 308. Therefore, the vibration of the moving member 308 cannot be smoothly transmitted to the non-compressive fluid, so that the dynamic spring constant of the engine mount is not sufficiently reduced and the vibration isolation effect is not satisfactorily achieved.
U.S. Pat. No. 4,793,599, discloses a power unit mounting apparatus using an actuator formed of a double coil, and U.S. Pat. No. 4,650,170 discloses a vibration isolator using an actuator formed of a solenoid. In these apparatuses, a smaller dynamic spring constant can be obtained by causing a moving member or a vibrating member (moving plate) to vibrate in a direction of phase in opposition to input vibrations, and a larger dynamic spring constant can be obtained by causing the moving member to vibrate in a direction in phase with input vibrations. In an engine mount such as that described above, vibrations from an engine are limited by selecting the smaller dynamic spring constant in a steady state during idling or the like, and engine shakes are prevented by selecting the larger dynamic spring constant during a transient operation. In these apparatuses, however, the stroke of movement of the moving member or vibrating member (moving plate) is constant. It is therefore difficult to vary the vibration smoothing ability.
As is well known, the efficient of the voice coil 310 is higher if the gap between the moving coil 307 and the permanent magnet 309 is smaller. In the above-described engine mount, however, the moving member 308 is affixed to the sheet-like member 311. Therefore, if the sheet-like member 311 is bent with a displacement of the moving member 308, the center of the moving member 308 deviates from the axis L so that the gap is changed. Consequently, the gap cannot be sufficiently reduced, as a result, there is a need to increase the size of the voice coil 310 to compensate for a reduction in the efficiency of the voice coil 310, resulting in an increase in the overall size of the engine mount.
Also, Japanese Patent Unexamined Publication No. 4-312229 discloses a fluid-enclosed vibration isolator for an engine mount. In this vibration isolator, an actuator is not inserted in alignment with a vibration isolating rubber member, and vibration isolation is made with a smaller operating energy. For energy saving, the actuator of this vibration isolator is also arranged as a voice coil type. However, it use an expensive rare earth magnet, that is, it was not suitably designed to reduce the manufacturing cost of the engine mount.